1. Field of the Invention
The present invention relates to a monolithic ceramic electronic component, and more particularly to a large monolithic ceramic electronic component for use in a middle and high voltage range.
Further, the present invention relates to a monolithic ceramic electronic component, and more particularly to a monolithic ceramic electronic component which includes plural internal electrodes disposed so as to be opposed to each other through ceramic layers in a ceramic element.
2. Description of the Related Art
Conventionally, as a smoothing condenser for electric cars, electrolyte condensers and film capacitors have been used. The electrolyte condensers have high ESR, and heat is evolved when a large current flows. To prevent the evolution of heat, it is necessary to provide a high capacitance capacitor with a capacitance of fifty to sixty thousand xcexcF. In general, the smoothing condenser comprises about 4 condensers each having an outside diameter of 8*15 cm and a capacitance of about 3000 xcexcF. Thus, the smoothing condenser has the problem that it is very large in size. Further, the electrolyte condenser has the problem that periodic maintenance is required, since the dry up of an electrolyte occurs. On the other hand, the film capacitor has a smaller ESR than the electrolyte condenser, and the capacitance is in the rage of 400 to 1000 xcexcF. However, since films are used as dielectric, the dielectric constant is low. Accordingly, the film capacitor comprises about two capacitors each having a size of 8*15 cm, which causes the problem that it is large in size. Further, since the film capacitor has a low resistance to heat, it is necessary further to increase the size of the film capacitor, when a large capacitance is rendered to the film capacitor.
FIGS. 3A and B are plan and front views each showing a monolithic ceramic capacitor 1 which is one example of a conventional monolithic ceramic electronic components interesting to the present invention.
As shown in FIGS. 3A and 3B, the monolithic ceramic capacitor 1 comprises a capacitor body 2 having a rectangular shape specified by a longitudinal dimension L, a widthwise dimension W, and a thickness-wise dimension T, and first and second external electrodes 3 and 4 formed on the opposite end-faces in the longitudinal direction of the capacitor body 2.
The capacitor body 2 comprises plural dielectric layers 5 which are laminated in the state that they are extended in parallel to the plane specified by the longitudinal dimension L and the widthwise dimension W, and plural sets of internal electrodes 6 which are opposed to each other through specific one of the dielectric layers 5, respectively.
As to the plural sets of internal electrodes 6, the internal electrodes electrically connected to the first external electrodes 3 and the internal electrodes electrically connected to the second external electrode 4 are alternately disposed. The internal electrode 6 shown by exploding a part of the capacitor body 2 in FIG. 3A is electrically connected to the first external electrodes which is seen in it""s profile shown by the broken line. The internal electrode 6 electrically connected to the second external electrode 4 is symmetrical with the illustrated internal electrode 6.
Conventionally, in general, such a monolithic ceramic capacitor 1 is designed so that the capacitor body has a dimensional relation between the longitudinal dimension L, the widthwise dimension W, and the thickness-wise dimension T of L greater than W and Wxe2x89xa62T.
The recent market demands that such monolithic ceramic capacitors 1 as described above should have a high capacitance and be suitable for high voltage uses.
As means for meeting the above demand, it is suggested that for the monolithic ceramic capacitor 1, the lamination number of the internal electrodes 6 and the thickness of the dielectric layers 5 should be increased.
However, if such a means is employed, the dielectric layers 5 each need to have a thickness of 20 xcexcm or more, so that the monolithic ceramic capacitor 1 may be made suitable for use in a middle and high voltage range of a rated voltage of 250V or higher. Therefore, to attain a large capacitance of 1 xcexcF or more, the lamination number of the internal electrodes 6 becomes very large, and thereby, the thickness-wise dimension T of the capacitor body 2 has to be remarkably increased.
Therefore, when fired to produce the capacitor body 2, ceramics constituting the dielectric layers 5 are insufficiently sintered or are unstable. That is, pores are ready to be formed in the dielectric layers 5, the internal electrodes 6 are insufficiently sintered, and the sintering state of the internal electrodes 6 tends to have dispersions. As a result, the initial characteristics of the obtained monolithic ceramic capacitor 1 are deteriorated. That is, possibly, delamination occurs, the breakdown voltage (BDV) is reduced, and cracks are readily caused, due to the electrostriction. Also, in some cases, the reliability such as the high temperature service-life or the like may be reduced.
As means for rendering a high capacitance to the monolithic ceramic capacitor 1, it is proposed that the effective area of the internal electrodes 6 is increased by increasing the longitudinal dimension L and the widthwise dimension W of the capacitor body 2.
However, even though the effective area of the internal electrodes 6 is increased by relatively increasing the longitudinal dimension L and the widthwise direction W of the capacitor body 2 as described above, this means only taken is the same that practically no measures for improving the BDV of the monolithic ceramic capacitor 1 are taken. Accordingly, the monolithic ceramic capacitor 1 when it is applied in a middle and high voltage range encounters problems of BDV or the like.
Chip monolithic ceramic capacitors, which are typical monolithic ceramic electronic components, are produced as follows. Plural internal electrodes 52 are disposed so as to be opposed to each other through ceramics (ceramic layers) 51, and the one-ends of the internal electrodes 52 are led-out alternately to the different end-faces of the ceramic element 54. On the opposite end faces of the ceramic element 54, a pair of external electrodes 53, 53 are disposed so as to be connected to the internal electrodes 52, as shown in FIG. 9, FIGS. 10A, 10B, and 10C, for example.
In the case that the monolithic ceramic capacitor having a structure as shown in FIGS. 9 and 10, which is a product for use in a middle and high voltage range, it is not necessarily easy to secure high withstanding voltage properties. It is needed to develop monolithic ceramic capacitors with a high reliability, having a high breakdown voltage and excellent withstanding voltage properties.
The above-description, not limited to monolithic ceramic capacitors, are also true of monolithic ceramic electronic components such as varistors, inductors, and so forth.
For the purpose of enhancing the breakdown voltages of such monolithic ceramic electronic components as described above, the following methods are ordinarily suggested.
(1) a method of increasing the thickness of the element (the distance between the opposed electrodes through the ceramic layers(thickness-wise distance)), and
(2) a method of rendering the internal electrodes such an electrode structure that plural series connection capacitances are formed.
However, the breakdown voltage has the tendency that it is dominated by the degree of electric fields concentrated onto the edge portions (52a in FIG. 10A) of the internal electrodes 52. In the case of the above-described methods (1) and (2) applied, it is practically difficult to enhance the breakdown voltage sufficiently, since the electric fields are concentrated onto the edge portions (peripheries and corners) of the internal electrodes 52.
Accordingly, to relax the electric field concentration onto the edge portions 52a of the internal electrodes 52, it is further necessary to devise the shape and-size of the internal electrodes 52 and the lamination-form thereof. Thus, the internal structure of the ceramic element becomes complicated, which causes the problem that the manufacturing expenditure is increased.
Accordingly, it is an object of the present invention to provide a monolithic ceramic electronic component which can solve the above-described problems, especially those of BDV.
Also, it is a further object of the present invention to provide a monolithic ceramic electronic component which can solve the above-described problems, and has excellent withstanding voltage properties even though it is a large produce, not requiring a complicated structure.
Further, it is a still further object of the present invention to provide a monolithic ceramic capacitor as a smoothing capacitor attached between an power supply and an inverter of an electric car.
The present invention is intended for a monolithic ceramic electronic component which comprises an electronic component body having a rectangular shape specified by a longitudinal dimension, a widthwise dimension, and a thickness-wise dimension, and first and second external electrodes formed on the opposite end-faces in the longitudinal direction of the electronic component body, respectively, the electronic component body comprising plural dielectric layers laminated in the state that the layers are extended in parallel to a plane specified by the longitudinal dimension and the widthwise dimension of the electronic component body, and plural sets of internal electrodes opposed to each other through a specific dielectric layer, respectively, the internal electrodes of the plural sets of internal electrodes electrically connected to the first external electrode and the internal electrodes thereof electrically connected to the second external electrode being alternately disposed. To solve the above-described technical problems, characteristically, both of the longitudinal dimension and the widthwise dimension of the electronic component body are at least four times the thickness-wise dimension thereof, respectively.
The present invention has been devised, paying attention to the relation between the longitudinal, widthwise, and thickness-wise dimensions, and BDV of an electronic component body. The BDV was evaluated by varying the longitudinal, widthwise, and thickness-wise dimensions. As a result, as described above, it has been found that the BDV is enhanced by setting both of the longitudinal and widthwise dimensions to be at least four times the thickness-wise dimension.
Further, as a result of the evaluation of the BDV by varying the longitudinal, widthwise, and thickness-wise dimensions of such an electronic component body as described above, it has been found that the BDV is further enhanced by setting the widthwise dimension to be greater than the longitudinal dimension. Thus, according to the present invention, preferably, the widthwise dimension of the electronic component body is set to be greater than the longitudinal dimension.
The present invention is applied especially advantageously in the case of such a large monolithic ceramic electronic component that both of the longitudinal and widthwise dimensions of the electronic component body are at least 10 mm, respectively, or the static capacitance is at least 1xcexcF, the thickness of each dielectric layer between the opposed internal electrodes is at least 20 xcexcm, and moreover, the rated voltage is at least 250V.
Further, the present invention is also applied especially advantageously in the case that the internal electrodes contain a base metal.
To achieve the above-described object, the inventors examined and investigated the internal structures of monolithic ceramic electronic components. The following information was obtained.
(1) In conventional monolithic ceramic electronic components such as monolithic ceramic capacitors, ordinarily, the thicknesses t of the internal electrodes 52 (FIG. 10C) are about 1 xcexcm.
(2) The area (plan effective area (the length lxc3x97the width w (=the width w of the internal electrodes) of the overlapping portions) of the overlapping portions 62 (FIG. 10A) of the internal electrodes is up to 5000 times the cross sectional area (the thickness txc3x97the width w of the respective internal electrodes 52) obtained by cutting the internal electrodes 52 (FIG. 10C) perpendicular to the leading-out direction of the internal electrodes.
(3) The ratio of the plan effective area of the internal electrodes to the cross sectional area thereof exerts an influence over the withstanding voltage properties.
The inventors have carried out further experiment and investigation based on the above information, and have completed the present invention.
That is, according to a second aspect of the present invention, there is provided a monolithic ceramic electronic component which has the configuration that plural internal electrodes are disposed in a ceramic element in the form that the plural internal electrodes are opposed to each other through ceramic layers, and the one-ends of the plural internal electrodes are led-out alternately to the different side-faces of the ceramic element, the area of the overlapping portions of the internal electrodes, viewed in the plan thereof, being at least 10000 times the cross sectional area per internal electrode layer obtained by cutting the internal electrodes perpendicular to the leading-out direction of the internal electrodes.
As described above, by setting the area (plan effective area) of the overlapping portions of the internal electrodes, viewed in the plan thereof, to be at least 10000 times the cross sectional area per internal electrode layer obtained by cutting the internal electrodes perpendicular to the leading-out direction of the internal electrodes, the concentration of electric fields onto the edge portions of the internal electrodes can be relaxed to improve the withstanding voltage properties.
The present invention may be applied to monolithic ceramic electronic components of a so-called stack type, which are formed by stacking plural ceramic elements.
Preferably, the monolithic ceramic electronic component is operable in a middle and high voltage range of a rated voltage of 250V or higher.
When the present invention is applied to a monolithic ceramic electronic component for use in a middle and high voltage range of a rated voltage of 250V or higher where the withstanding voltage properties readily cause problems, advantageously, the withstanding voltage properties can be securely enhanced to such a degree that they have no problems for practical use, without the thickness of the element being increased.
Also preferably, the ceramic element has a dimension in parallel to the leading-out direction of the internal electrodes of 10 mm or more.
As to the monolithic ceramic electronic component in which the ceramic element has a dimension in parallel to the leading-out direction of the internal electrodes of 10 mm or more, especially, the withstanding voltage properties tend to cause problems. When the present invention is applied to such a large monolithic ceramic electronic component, advantageously, the withstanding voltage properties can be securely enhanced to such a degree that they have no problems for practical use.
Further, the monolithic ceramic electronic component is preferably a monolithic ceramic capacitor.
As to monolithic ceramic capacitors, especially if they are large products, the withstanding voltage properties readily cause problems. However, by applying the present invention, a monolithic ceramic capacitor having excellent withstanding voltage properties can be obtained.